The author of a biography of Indira Gandhi was last night bracing herself for a flurry of writs after members of the Gandhi dynasty expressed outrage at the book's contents - and the claim that Indira's late son Sanjay had "murdered" several of his enemies.

Feroze Varun Gandhi, the late Indian prime minister's 21-year-old grandson, yesterday told the Guardian that his family was considering legal action against its American author, Katherine Frank, and its publisher, HarperCollins.

The family had been deeply stung by allegations that his father Sanjay had "eliminated" opponents during India's infamous Emergency between 1975 and 1977, he said. The biography claims that Sanjay - Mrs Gandhi's younger son, who died in an air crash in 1980 - ordered his "hitmen" to "liquidate" several "human targets". They included at least two underworld figures, one of whom was dumped in a river.

The book, Indira: The Life of Indira Nehru Gandhi, was published this month. It has provoked controversy in India for exploring Mrs Gandhi's "problematic" sex life - and for floating the possibility that her husband Feroze might have had an affair with her mother, Kamala Nehru, in the 1930s, before their marriage. The claim was made in a scurrilous poster campaign in the Indian town of Allahabad.

"There are very serious allegations in the book and she better be able to prove them," Mr Gandhi said yesterday. "If the people who have talked to her retract their evidence then she is going to be in deep water. We have not yet decided our course of action. She can stick by her story. I hope the supreme court [in Delhi] will be able to stick by her story."

The biography is also less than flattering about Sanjay Gandhi's widow Maneka - now a minister in India's Hindu nationalist-led government. It describes her behaviour as "erratic and uncontrolled" and says her "wild outbursts and tantrums" created a "pervasive atmosphere of uncertainty and anxiety" in the Gandhi household during a time of political turbulence.

Last night Frank defended her best-selling biography. She was not panicking at the prospect of a writ from the Gandhi family, she said. "If people want to sue me I can't do anything about it.

"Everything I say is sourced and footnoted. If it happened, it happened. I feel confident that my narrative will hold up and I have told the truth." She added: "I don't feel I have unjustly libelled anybody. I have tried to be scrupulously fair."

Frank, who lives in Britain, said reviewers had tended to ignore the new material in her book - including the revelation that Mrs Gandhi's early life had been blighted by tuberculosis. Instead, they had concentrated on her alleged affairs - with her father's private secretary, MO Mathai, and others.

"The only thing I did was to assess whether these rumours were credible," Frank said. "I included them because everyone in Indira Gandhi's circle knew about them. I find it highly unlikely she had any love affairs, though some people think she did."

It was "1,000% inconceivable" that Mrs Gandhi's husband had an affair with the woman who became his mother-in-law, she said, though they were emotionally close.

Last night a spokeswoman for HarperCollins in London said the company's representatives in India were trying to calm the controversy. She blamed "hysterical" accounts of the book, notably in a magazine, for stirring up trouble.